Although in vivo studies of normal and neoplastic breast tissue indicate that both growth and differentiation processes are hormonally influenced, much uncertainty exists regarding whether the response to hormones is mediated via a direct or indirect interaction of hormone(s) with mammary cells. Furthermore, even in those instances where direct responses to hormones have been observed in in vitro culture experiments, it is unknown whether a specified hormone interacts with one or all of the cellular types of which normal and neoplastic tissues are comprised. Our objective is to assess the peptide and steroid hormone receptor levels in the various cell types (epithelial, myoepithelial and stromal) of normal mammary gland and mammary tumors, to correlate receptor levels with biological responses and to explore the possibility that hormonal actions are the result of coordinated interaction of the various cell types of normal and tumor tissue.